The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Substrate processing systems typically include a processing chamber with one or more reaction volumes. A pedestal is typically located in the reaction volume. A substrate such as a semiconductor wafer is positioned on the pedestal. One or more process gases are delivered to the reaction volume using a showerhead and plasma may be struck in the reaction volume. A film such as a dielectric layer is formed on the substrate.
The showerhead may be used in a capacitively coupled plasma (CCP) reactor. The showerhead distributes process gases over the substrate and serves as a radio-frequency (RF) electrode to drive the plasma. The showerhead is typically made of a metallic material. A metal electrode substantially reduces or eliminates an electric field inside a gas plenum of the showerhead to prevent plasma formation inside the showerhead and premature activation of gases.
The showerhead for CCP reactors is typically made of aluminum and includes a face plate that is welded to a body. The face plate of the showerhead typically has a plurality of spaced gas holes to provide uniform gas distribution over an exposed surface of the substrate. RF voltage may be applied to the showerhead, to another electrode (such as the pedestal), or to both.
Aluminum showerheads work well with many process chemistries (or gas compositions). However, aluminum is not compatible with process gases that leach elements from metallic surfaces. In particular, chlorine-based chemistry tends to leach aluminum at operating temperatures exceeding 300° C. As a result of the chemical attack of the chlorine-containing gas on the metal surface of the showerhead, the metallic material may end up in the film deposited on the substrate. This is often detrimental to device fabrication on the substrate. For example, in some instances the metallic materials are dopants that can compromise integrated device operation.
Undesirable metal contamination can also occur during cleaning of the processing chamber. Since atomic fluorine is commonly used for cleaning, the face plate (the hottest area of the showerhead) reacts with the fluorine and forms aluminum fluoride. When a thickness of aluminum fluoride increases with time, showerhead surface properties (e.g., roughness, conductivity and emissivity) change. As a result, the deposition process can drift and is also at high risk for particle contamination.
The design of the showerhead also does not permit cleaning inside of the showerhead (because it is welded) while the showerhead is installed in the processing chamber. In extreme cases, fluorination of the aluminum surface inside of the gas holes in the face plate may change the diameter of the holes and alter the uniformity of gas flow.
Some metallic materials such as aluminum become softer at higher temperatures (over 400° C.) and the face plate of the showerhead may start to droop. This can cause changes in gas flow and plasma density distribution.